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A method of focusing a telecentric imaging system (30), particularly as a
part of a measuring machine (10) includes measuring an image of a feature
(25) of an object (24) through the telecentric imaging system (30) in a
telecentric operating mode and measuring an image of the feature (25) of
the object (24) through the telecentric imaging system (30) in a
non-telecentric operating mode. A value is acquired characterizing a
function by which the size of the imaged feature varies in the
non-telecentric mode with the relative displacement of the object (24)
through the depth of field (D). The measures of the image of the feature
(25) of the object (24) in the telecentric and non-telecentric modes are
related to each other and to the acquired value as an estimate of a
relative displacement of the object (24) from the best focus position.

Inventors:

Lawson; David E.; (Webster, NY); Bloch; Stephanie M.; (Rush, NY)

Applicant:

Name

City

State

Country

Type

Quality Vision International, Inc.

Rochester

NY

US

Family ID:

1000002253721

Appl. No.:

15/299523

Filed:

October 21, 2016

Related U.S. Patent Documents

Application Number

Filing Date

Patent Number

62264924

Dec 9, 2015

Current U.S. Class:

1/1

Current CPC Class:

G06T 7/0044 20130101; G02B 21/365 20130101

International Class:

G02B 21/36 20060101 G02B021/36; G06T 7/00 20060101 G06T007/00

Claims

1. A method of focusing a telecentric imaging system of a measuring
machine comprising steps of: measuring an image of a feature of an object
through the telecentric imaging system in a telecentric operating mode in
which a size of the imaged feature remains substantially constant with
relative displacement of the object through a depth of field; measuring
an image of the feature of the object through the telecentric imaging
system in a non-telecentric operating mode in which the size of the
imaged feature varies as a function of the relative displacement of the
object through the depth of field; acquiring a value characterizing the
function by which the size of the imaged feature varies with the relative
displacement of the object through the depth of field; relating the
measures of the image of the feature of the object in the telecentric and
non-telecentric modes to each other and to the acquired value
characterizing the function by which the size of the imaged feature
varies with the relative displacement of the object through the depth of
field as an estimate of a relative displacement of the object from the
best focus position; and relatively displacing the object with respect to
the telecentric imaging system by the estimate of the relative
displacement of the object from the best focus position.

2. The method of claim 1 including steps of relatively displacing the
object within the depth of field by a known amount, and measuring an
image of the feature of the object through the telecentric imaging system
in the non-telecentric at the relatively displaced position of the
object.

3. The method of claim 2 in which the steps of measuring an image of the
feature of the object through the telecentric imaging system include
measuring the sizes of the imaged features, and including a step of
comparing a measured variation in the sizes of the imaged features
between the relatively displaced positions of the object in the
non-telecentric mode to the known relative displacement of the object
within the depth of field for defining the value characterizing the
function by which the size of the imaged feature varies with the relative
displacement of the object through the depth of field.

4. The method of claim 1 in which the step of acquiring a value
characterizing the function by which the size of the imaged feature
varies includes accessing a predefined value.

5. The method of claim 4 in which the predefined value is one of a
plurality of predefined values stored in a lookup table or mathematical
representation, and the predefined values vary with radial position
within the image field to represent different rates of change in
magnification with the relative displacement of the object in the
non-telecentric mode.

6. The method of claim 4 including a further step of measuring an image
of the feature of the object through the telecentric imaging system in
the non-telecentric mode at the relatively displaced position of the
object as a first estimate of the best focus position.

7. The method of claim 6 in which the steps of measuring an image of the
feature of the object through the telecentric imaging system include
measuring the sizes of the imaged features, and including a step of
comparing a measured variation in the sizes of the imaged features
between the relatively displaced positions of the object in the
non-telecentric mode to the estimated relative displacement through which
the object is relatively displaced to acquire a refined value
characterizing the function by which the size of the imaged feature
varies with the relative displacement of the object through the depth of
field.

8. The method of claim 7 including further steps of relating the size of
the imaged feature in the telecentric mode and the size of the imaged
feature in the non-telecentric mode at the relatively displaced position
to each other and to the refined value characterizing the function by
which the size of the imaged feature varies with the relative
displacement of the object through the depth of field as a finer estimate
of a relative displacement of the object from the best focus position,
and relatively displacing the object with respect to the telecentric
imaging system by the finer estimate of the relative displacement of the
object from the best focus position.

9. The method of claim 1 including steps of emitting a first light beam
for illuminating the object whereby the image of the feature of the
object in the telecentric mode is formed with a nominal wavelength of the
light beam that is within a range of wavelengths at which the telecentric
imaging system is corrected to maintain telecentricity, and emitting a
second light beam for illuminating the object whereby the image of the
feature of the object in the non-telecentric mode is formed with a
nominal wavelength that is outside the range of wavelengths at which the
telecentric imaging system is corrected to maintain telecentricity.

10. The method of claim 9 in which the first and second light beams are
emitted from a common light source and in which the step of emitting the
second light beam includes changing an amount of current to the light
source.

11. The method of claim 9 in which the wavelength outside the corrected
range subjects the image of the feature of the object to a chromatic
aberration and in which an axial component of the chromatic aberration
changes focal lengths of optics within the telecentric imaging system
such that an aperture stop of the imaging system is no longer located at
a back focus as required to locate an entrance pupil of the imaging
system at infinity.

12. The method of claim 11 in which the wavelength at which the
telecentric imaging system is operated in a non-telecentric mode is
sufficiently close to the range of wavelengths at which the telecentric
imaging system is designed to operate in a telecentric mode so that a
primary effect is a displacement of image points within the image field
without a significant loss in sharpness affecting an ability to measure
image feature size within a region at which the depths of field in both
the telecentric and non-telecentric modes overlap.

13. A focus system for focusing a telecentric imaging system of a
measuring machine comprising: a camera including a telecentric imaging
system and a detector on which images relayed by the telecentric imaging
system are formed; the camera being relatively movable with respect to an
object intended for measurement along an optical axis of the telecentric
imaging system; the telecentric imaging system being operable in a
telecentric mode in which a size of an imaged feature of the object
remains substantially constant with relative displacement of the object
through a depth of field; the telecentric imaging system also being
operable in a non-telecentric mode in which the size of the imaged
feature varies as a function of the relative displacement of the object
through the depth of field; the telecentric imaging system being
adjustable between the telecentric and non-telecentric modes; a processor
arranged for extracting size measurements of the imaged feature from
images captured by the detector in both the telecentric and
non-telecentric modes; and the processor also being arranged for relating
the measures of the size of the imaged feature in the telecentric and
non-telecentric modes to each other and to the function by which the size
of the imaged feature varies with the relative displacement of the object
through the depth of field as an estimate of a relative displacement of
the object from a best focus position.

14. The focus system of claim 13 further comprising: an illuminator for
illuminating the object with light that is collected by the telecentric
imaging system for imaging the object onto the detector, wherein the
illuminator is arranged for emitting light having a first peak wavelength
within a range of wavelengths at which the telecentric imaging system is
corrected for operating the telecentric imaging system in the telecentric
mode, and the illuminator is arranged for emitting light having a second
peak wavelength outside the range of wavelengths at which the telecentric
imaging system is corrected for operating the telecentric imaging system
in the non-telecentric mode

15. The focus system of claim 14 in which the illuminator is adjustable
between emitting light at the first peak wavelength and emitting light at
the second peak wavelength for adjusting the telecentric imaging system
between the telecentric and non-telecentric modes.

16. The focus system of claim 13 in which the processor is arranged for
accessing stored data concerning the function by which the size of the
imaged feature varies with the relative displacement of the object
through the depth of field.

17. The focus system of claim 16 in which the stored data differs by
radial distance from the optical axis to accommodate certain distortions
of the image feature in the non-telecentric mode.

18. The focus system of claim 13 in which the processor operates
according to an algorithm by which the machine is operated for capturing
images of the feature of the object through the telecentric imaging
system in both the telecentric and non-telecentric operating modes at the
same relative position between the camera and the object, relatively
displacing the object with respect to the camera by a known distance
along the optical axis within the depth of field, and capturing another
image of the feature of the object in the non-telecentric mode at the
relatively displaced position of the object with respect to the camera.

19. The focus system of claim 18 in which the processor operates
according to a further algorithm for measuring sizes of the feature
within the captured images, and comparing a variation in the measured
sizes of the imaged features between the relatively displaced positions
of the object in the non-telecentric mode to the known relative
displacement of the object within the depth of field for defining a value
characterizing the function by which the size of the imaged feature
varies with the relative displacement of the object through the depth of
field.

20. The focus system of claim 13 in which the processor is arranged to
direct the relative displacement of the object with respect to the camera
by the estimate of the relative displacement of the object from a best
focus position.

21. A method of measuring a best focus position of a telecentric imaging
system comprising steps of: measuring an image of a feature of an object
through the telecentric imaging system in both a telecentric operating
mode and a non-telecentric operating mode in which a size of the imaged
feature remains substantially constant with relative displacement of the
object through a depth of field in the telecentric mode and in which the
size of the imaged feature varies as a function of the relative
displacement of the object through the depth of field in the
non-telecentric mode; and relating the measures of the images of the
feature of the object in the telecentric and non-telecentric modes to
each other and to the function by which the size of the imaged feature
varies with the relative displacement of the object through the depth of
field as an estimate of a relative displacement of the object from the
best focus position.

22. The method of claim 21 in which the image of the feature of the
object in the telecentric mode is formed with a nominal wavelength that
is within a range of wavelengths at which the telecentric imaging system
is corrected to maintain telecentricity and in which the image of the
feature of the object in the non-telecentric mode is formed with a
nominal wavelength that is outside the range of wavelengths at which the
telecentric imaging system is corrected to maintain telecentricity.

23. The method of claim 22 in which the step of measuring an image of a
feature of an object through the telecentric imaging system includes a
sub-step of changing a peak wavelength output from a light source from
the nominal wavelength that is within a range of wavelengths at which the
telecentric imaging system is corrected to maintain telecentricity to the
nominal wavelength that is outside the range of wavelengths at which the
telecentric imaging system is corrected to maintain telecentricity.

24. The method of claim 23 in which the sub-step of changing the peak
wavelength includes changing an amount of current to a LED light source.

25. The method of claim 22 in which the wavelength outside the corrected
range introduces a chromatic aberration into the image of the feature of
the object and in which an axial component of the chromatic aberration
changes focal lengths of optics within the telecentric imaging system
such that an aperture stop of the imaging system is no longer located at
a back focus as required to locate an entrance pupil of the imaging
system at infinity.

26. The method of claim 21 including steps of relatively displacing the
object within the depth of field by a known amount, and measuring an
image of the feature of the object through the telecentric imaging system
in the non-telecentric at the relatively displaced position of the
object.

27. The method of claim 26 in which the steps of measuring an image of
the feature of the object through the telecentric imaging system include
measuring the size of the imaged feature, and including a step of
comparing a measured variation in the sizes of the imaged features
between the relatively displaced positions of the object in the
non-telecentric mode to the known relative displacement of the object
within the depth of field for defining a value characterizing the
function by which the size of the imaged feature varies with the relative
displacement of the object through the depth of field.

28. The method of claim 25 in which local magnification varies with
object distance from the best focus position as a result of an axial
component of the chromatic aberration, and magnification changes at the
best focus position as a result of a lateral component of the chromatic
aberration.

29. The method of claim 28 in which the step of relating provides for
identifying an amount of relative displacement of the object whereby an
image of the feature of the object measured in the non-telecentric mode
is expected to at least approximately exhibit the magnification change
induced by the lateral component of the chromatic aberration.

30. The method of claim 21 in which the step of relating includes
accessing one or more predefined values characterizing the function by
which the size of the imaged feature varies with the relative
displacement of the object through the depth of field.

31. The method of claim 30 in which the predefined values are stored in a
lookup table and vary with radial position within the image field to
represent different rates of change in magnification with the relative
displacement of the object in the non-telecentric mode.

Description

TECHNICAL FIELD

[0001] The invention relates to focusing systems for telecentric imaging
systems, used for example in image-based measuring machines, and to
techniques for estimating positions of best focus based on measurements
taken through the telecentric imaging systems.

BACKGROUND OF THE INVENTION

[0002] In imaging systems, image sharpness can vary with departures from
the best focus position. As such, the best focus position of imaging
systems is often determined by moving the position of the object relative
to the imaging system until the sharpest image is formed. Typically,
image contrast is used as a measure of sharpness to identify the best
focus position at the peak contrast.

[0003] U.S. Pat. No. 7,812,971, which is assigned in common with this
application, features an autofocus system for a machine vision system
that scans along an optical axis collecting a plurality of image frames
at a plurality of different wavelengths to determine the positions of
maximum contrast for each wavelength. The maximum contrast measurements
can be combined based on the expected displacements among the wavelengths
to determine a position of best focus. Alternatively, contrast
measurements for the different wavelengths at a single axis position can
be fit to the contrast value plots of the different wavelengths over a
range of axis positions for identifying the best focus position.

[0004] In optical systems with large depths of field, e.g., having a large
f-number, the variations in sharpness in the vicinity of the best focus
position tend to be more gradual, which reduces the accuracy and
precision with which the best focus position can be found. Departures
from the best focus positions can often be tolerated for optical systems
with large f-numbers because the variation in sharpness is so gradual.
However, in such systems that are arranged to switch between
magnifications, the departure from best focus workable at a lower
magnification can be beyond the depth of field at a higher magnification,
leaving no measurable image from which a further focus adjustment can be
made. Making separate focus adjustments between low and high
magnification imaging can be time consuming and add variability between
the measurements.

[0005] Measures of contrast, which often involve pixel-to-pixel intensity
comparisons, differ from the usual measures for which the measuring
machines are designed and require additional processing algorithms and
other capabilities. Unless the object subject to imaging is matched to a
given choice of algorithm, the efficacy of the contrast measurement can
vary considerably. Thus, inconsistent focusing results can be associated
with a range of different objects that are the subject of the intended
measurement.

SUMMARY OF THE INVENTION

[0006] Among the embodiments described is a focusing system for an optical
measuring machine having a telecentric imaging system. Preferably, the
focusing system operates substantially independently of the f-number or
depth of field of the telecentric imaging system. For purposes of
focusing, the telecentric imaging system is operated in a non-telecentric
mode, such as by inducing chromatic aberration, and one or more
associated measures of distortion are converted into a measure of object
displacement from the best focus position. The telecentric imaging system
in its usual mode of operation is arranged for measuring the sizes of
features of objects as imaged within an image field, and distortion in
the non-telecentric mode can also be measured as changes in feature size
within the image field. Thus, the regular measuring capabilities of the
measuring machine can be exploited for determining the best focus
position.

[0007] For inducing the chromatic aberration, the illuminator of the
optical measuring machine is preferably operated at a wavelength that is
outside the range of wavelengths at which the telecentric imaging system
is corrected. Within the wavelength range at which the telecentric
imaging system is corrected, the imaging system exhibits telecentric
behavior at least with respect to object space. Wavelengths beyond the
range at which the telecentric imaging system is corrected introduce
chromatic aberrations and corresponding departures from telecentricity.
For example, an axial component of chromatic aberration changes the focal
lengths of optics within the telecentric imaging system such that the
aperture stop of the telecentric imaging system is no longer located at
the back focus as required to locate the entrance pupil at infinity.
While a lateral component of chromatic aberration also tends to change
the magnification, more significantly for the purposes herein,
magnification also varies with object distance as a result of the
departure from telecentricity. The effects of the departure in wavelength
tend to remain radially symmetric, and as such, the effects can be
defined as a function of distance from the center of the image field or
optical axis. The changes in magnification can be referred to more
generally as changes in distortion to encompass both the first order
change in magnification and higher order changes in magnification with
distances from the field center. The higher order changes in
magnification can affect imaged feature shape in addition to imaged
feature size.

[0008] Preferably the wavelength at which the measuring machine is
operated in a non-telecentric mode is still sufficiently close to the
range of wavelengths at which the telecentric imaging system is designed
to operate so that the primary effect is a displacement of image points
within the image field, classifiable as a radial distortion, without also
resulting in a loss in sharpness affecting the ability to measure image
feature size. While sharpness could vary somewhat through the depth of
field as a function of the object distance from the best focus position,
imaged points and edges preferably remain sufficiently sharp so that
measurements of image height can be made throughout an overlapping depth
of field in both the telecentric and non-telecentric modes.

[0009] Consistent with the fundamental expectations of telecentricity,
image height remains substantially constant through the depth of field.
In other words, magnification does not significantly vary with object
distance through the depth of field. Thus, regardless of the object
position in the depth of field, i.e., the object's relative departure
from the best focus position, substantially the same image height is
measured by the optical measuring machine operating in its telecentric
mode.

[0010] In contrast, measurements of image height taken with a wavelength
beyond the range of correction, tend to vary first as a result of a
lateral chromatic aberration evident as an initial change in
magnification even at the best focus position, and second, more
significantly, as a result of an axial chromatic aberration resulting in
a departure from telecentricity evident as a further variation in
magnification as a function of the object's relative departure from the
best focus position.

[0011] Thus, the image height of a measured object remains substantially
invariant with departures from the best focus position of the optical
measuring machine operating in the telecentric mode. However, the image
height of the same measured object varies as a function of departures
from the best focus position of the optical measuring machine operating
in the non-telecentric mode.

[0012] The two measures of image height can be correlated in various ways
to estimate the object's relative departure from the best focus position.
For example, a first measure of image height can be taken in the
telecentric mode to establish a baseline image height, which is expected
to remain constant regardless of the position of the object within the
depth of field. A second measure of image height can be taken at the same
position of the object within the depth of field but in the
non-telecentric operating mode, and a first height difference between the
telecentric and non-telecentric measurement modes can be recorded. While
remaining in the non-telecentric operating mode, a third measure of image
height can be taken at a relatively displaced position of the object
within the depth of field. The amount of displacement can be
predetermined or measured. A second height difference between the
telecentric first measurement and the non-telecentric third measurement
can be recorded. Given the two height differences over a known
displacement, a first linear approximation of the further relative
displacement of the object to the best focus position can be determined
where the height difference between the telecentric and non-telecentric
measures is reduced to zero. The linear approximation reflects an
expected proportional change in local magnification with object
displacement through the depth of field.

[0013] A better approximation of the object displacement from the best
focus position can be obtained by accounting for the change in image
height associated with the lateral component of the chromatic aberration,
which changes the magnification at the best focus position. Thus, instead
of solving for a displacement at which the difference between the
telecentric and non-telecentric measurements reduces to zero, the linear
approximation can include an offset representing the difference in image
height expected at the best focus position. However, where the departure
from telecentricity is effected by a wavelength shift just beyond the
corrected range of wavelengths, the difference in image height between
the two modes at the best focus position is expected to be minimal and in
many instances can be ignored.

[0014] For a given telecentric imaging system and a given wavelength
outside the range of correction for operating the telecentric imaging
system in a non-telecentric mode, local values of distortion can be
predicted or measured throughout the image field and over the depth of
field. Assuming radial symmetry, the distortion values only need to be
taken over a domain of image heights as radial distances from the field
center. While distortion can be represented in a variety of ways, the
distortion values can be presented as percentages calculated by the
difference in object height between the telecentric and non-telecentric
modes (h.sub.1-h.sub.0) divided by the object height of the telecentric
mode (h.sub.0). A single percentage can represent changes in
magnification that do not vary throughout the image field. Discrete
distortion values can be associated with different image heights or a
distortion curve can also be fit to the values.

[0015] Assuming that the distortion at any particular image height varies
proportionally to the relative displacement of the object through the
depth of field, then the change in distortion at each image height over a
change in displacement along the depth of field can be represented by a
single rate or slope. Since a change in the distortion at any point in
the image field can be represented as a local change in magnification

( h 2 h 0 - h 1 h 0 ) , ##EQU00001##

the rate or slope corresponds to a change in magnification with respect
to a change in object displacement. In addition, each image height can
also be associated with a magnification offset attributable to the
lateral chromatic aberration, which could remain approximately constant
over the image field or vary with field position representing a higher
order change in magnification.

[0016] Having characterized the distortion effects of the non-telecentric
mode throughout the field of view and over the depth of field, a single
pair of measurements taken in the telecentric and non-telecentric modes
at a particular image height and object displacement can be associated
with a predicted or previously measured slope, corresponding to the
expected rate of change of the local magnification with relative object
displacement, and, if desired, the pair of measurements can be further
associated with a local magnification offset associated with the lateral
component of the chromatic aberration, to estimate an amount of
displacement to the best focus position. That is, by measuring the
relative magnification contributed by the non-telecentric mode at a
measured image height and knowing both the rate of change of the local
magnification with object displacement and the expected local
magnification offset at the best focus position, a close estimate can be
made of the object displacement required to reach the best focus
position.

[0017] As a range finder or first approximation, an average slope value,
representing the rate of change in magnification with object displacement
within the depth of focus can be predicted from modeling or can be
empirically determined. That is, although the actual rate may vary with
radial position in the field due perhaps in part to higher order
distortion effects, an average slope value together with a single pair of
measurements in the telecentric and non-telecentric modes can provide an
estimate of the object displacement from the best focus position. While
this estimate may be of sufficient accuracy in certain applications to
effect a final focus adjustment or to determine an approximate distance
of the object, the estimate could also be used as a first stage of focus
adjustment.

[0018] After relatively displacing the object by the estimated amount,
another non-telecentric measurement of image height can be made. Now,
with two measures of object feature height taken in the non-telecentric
mode over a known range of object displacement together with the base
measurement taken in the telecentric mode, a more accurate determination
of the rate of change of object height difference with displacement
(representative of the change in magnification with respect to the change
in displacement) can be calculated as described above. The first
approximated relative displacement places the object much nearer to the
best focus position, and the more accurately measured slope following the
first displacement can be used to more finely estimate a further relative
displacement to relatively move the object closer to the best focus
position.

[0019] If desired, the process of taking additional non-telecentric
measurements and relatively adjusting the object position can be repeated
until the non-telecentric measurement of image height approaches the
telecentric measurement of image height by any difference of the offset
expected as a result of lateral chromatic aberration.

[0020] An embodiment as a method of measuring a best focus position of a
telecentric imaging system includes measuring an image of a feature of an
object through the telecentric imaging system in both a telecentric
operating mode and a non-telecentric operating mode. The size of the
imaged feature remains substantially constant with relative displacement
of the object through a depth of field in the telecentric mode. However,
in the non-telecentric mode, the size of the imaged feature varies as a
function of the relative displacement of the object through the depth of
field. The measures of the images of the feature of the object in the
telecentric and non-telecentric modes are related to each other and to
the function by which the size of the imaged feature varies with the
relative displacement of the object through the depth of field as an
estimate of a relative displacement of the object from the best focus
position.

[0021] Another embodiment as method of focusing a telecentric imaging
system includes both measuring an image of a feature of an object through
the telecentric imaging system in a telecentric operating mode in which a
size of the imaged feature remains substantially constant with relative
displacement of the object through a depth of field and measuring an
image of the feature of the object through the telecentric imaging system
in a non-telecentric operating mode in which the size of the imaged
feature varies as a function of the relative displacement of the object
through the depth of field. A value characterizing the function by which
the size of the imaged feature varies with the relative displacement of
the object through the depth of field can be acquired in various ways,
including as a result of modeling the expected behavior of the
telecentric imaging system or as a result of earlier measurements. The
measures of the image of the feature of the object in the telecentric and
non-telecentric modes can be related to each other and to the acquired
value characterizing the function by which the size of the imaged feature
varies with the relative displacement of the object through the depth of
field as an estimate of a relative displacement of the object from the
best focus position. The object can be relatively displaced with respect
to the telecentric imaging system by the estimate of the relative
displacement of the object from the best focus position.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a diagrammatic view of a machine vision system
implementing the autofocus apparatus and method of this invention.

[0023] FIG. 2A is a diagram of an imaging system operating in a
telecentric mode.

[0024] FIG. 2B is a diagram of an imaging system operating in a
non-telecentric mode.

[0025] FIG. 3 is a graph showing image height differences between the
telecentric and non-telecentric modes over a range of displacements of an
object with respect to the telecentric imaging system within a depth of
field.

DETAILED DESCRIPTION

[0026] Certain embodiments are particularly applicable to telecentric
large-field-of-view optical systems and to such systems having large
depths of field, e.g., using lenses having high f-numbers. For example,
particular embodiments contemplate depths of field of 100 mm or more.
Within such depths of field, image sharpness, such as might be measured
by contrast, varies gradually through the depth of field, particularly
near the best focus position, so that the peak sharpness (e.g., highest
contrast) of the best focus position is difficult to distinguish from
nearby positions.

[0027] A focusing method as described herein does not rely on measures of
image sharpness to identify the best focus position. Instead,
conventional image height or size measurements are taken of an object
feature as seen through a telecentric imaging system that can be operated
in both telecentric and non-telecentric modes. In the telecentric mode,
the image height or size of the feature does not significantly vary
through the depth of field. That is, in accordance with the regular
expectations of a telecentric imaging system, magnification does not
significantly vary with object distance within the depth of field. As
such, the object feature is imaged by the telecentric imaging system at
substantially the same image height regardless of the position of the
object within the depth of field.

[0028] In the non-telecentric mode, the special requirements for
telecentricity are not met and magnification tends to vary with object
distance. However, the departure from telecentricity is not intended to
significantly reduce the local sharpness of the image. Beyond the normal
expectations regarding changes in magnification, image information is
largely geometrically displaced rather than lost, and the geometric
displacement, which varies as a function of object distance through the
depth of field, can be measured as a clue to the best focus position.
Even though local magnification changes with object distance in the
non-telecentric mode, the varying height or size of the object feature
within the image field preferably remains similarly measurable throughout
substantially the same depth of field present in the telecentric mode.
Any substantial differences between the two modes are preferably limited
to the ends of the depth of field, where sharpness has substantially
deteriorated.

[0029] Referring now to FIG. 1, a measuring machine in the form of machine
vision system indicated generally at 10 includes a camera 12 and an
illuminator 14 mounted on a carriage 16, which is moveable along a
vertical Z-axis on slides 18 and 20 as shown by arrow Z. The illuminator
14 is shown as a ring light having a central aperture through which
images are acquired and a surrounding annulus containing a plurality of
light sources, which are not individually shown. For example, the
illuminator 14 can contain selectively actuable, essentially
monochromatic light sources, such as a plurality of LED's (light-emitting
diodes) for illuminating an object 24 with monochromatic light.

[0030] The carriage 16 carrying the camera 12 is translatable along the
Z-axis to adjust distance between the camera 12 and the object 24. The
object 24, which is mounted on table 26, is similarly translatable along
orthogonal horizontal axes X and Y to align different portions of the
object with the field of view of the camera 16. Thus, the depicted
measuring machine 10 includes three axes of linear motion for moving the
camera 12 and the object 24 relative to each other. Similar results can
be obtained by dividing the axes differently between the camera 12 and
the object 24. In fact, one of the camera 12 and the object 24 can be
stationary and the other of the camera 12 and the object 24 can be
movable along any or all of the axes of motion. Although three
rectilinear axes of motion are shown, the axes can be oriented
differently and one or more rotational axes can be added or substituted
to provide additional orientations between the camera 12 and the object
24 as required for measuring or otherwise imaging features of the object
24.

[0031] The camera 12 includes a telecentric imaging system 30, which is
preferably in the form of a telecentric lens, which relays an image of
the object 24 to a sensor 32 located in an image field of the telecentric
imaging system 30. The sensor 32 can be a pixilated sensor such as a CCD
(charge-coupled device) sensor. The telecentric imaging system 30 is
preferably designed to reproduce an image of the object in a given
wavelength with limited aberrations to support measurements of the object
24 with a desired accuracy. In addition, the telecentric imaging system
30 is designed to be telecentric for the given wavelength at least in an
object space within a depth of field at which features of the object,
such as the feature 25, are reproduced with adequate definition for
measuring the image height or size of the feature with good precision
(i.e., the measures are repeatable to within a given tolerance).

[0032] Image height can be measured along a radius from the center of the
field and image size can be measured between any two points in the field.
Knowing the center of the field, image height can be measured from the
position of a single point in the field and can be more readily scaled or
otherwise accommodated to radial distortions. A processor 28, which
communicates with the camera 12, the illuminator 14, and the machine axes
of motion (e.g., displacement commands and measurements along the X, Y,
and Z axes), regulates sequences of operations including extracting size
measurements of the imaged feature 25 from images captured by the
detector 32 at two or more relative positions along the Z-axis of motion.

[0033] The illuminator 14 can be arranged to support various forms of
imaging from brightfield imaging, where light specularly reflected from
the object 24 enters the camera, to darkfield imaging where light must be
non-specularly reflected from the object to enter the camera. LEDs within
the illuminator 14 can be powered or controlled individually or in groups
or sectors, and the LEDs can be separately or collectively associated
with one or more lenses, such as a Fresnel lens to focus or otherwise
angularly direct their light toward the object 24.

[0034] Other illumination techniques could also be used including various
types of oblique or axial illumination as well as illuminators positioned
for through-the-lens illumination in which light from an illumination
source passes through an objective lens of the telecentric imaging system
30. Backlighting could also be used such as for measuring the silhouette
of the object 24.

[0035] Preferably, the light source is substantially monochromatic having
a nominal or peak wavelength for which the telecentric imaging system 30
is properly corrected to exhibit the desired level of sharpness and
telecentricity. In addition, the light source is preferably adjustable
for varying the peak wavelength emitted by the light source. For example,
the electrical current to the LEDs within the illuminator 14 can be
adjusted to shift the peak wavelength beyond the range at which the
telecentric imaging system is corrected for maintaining telecentricity.
Although the amount of wavelength shift with a change in current can vary
for different LED designs, a given change in wavelength can produce a
predictable and repeatable change in wavelength for a given LED design.
Even small shifts in peak wavelength in the range of 10 nanometers (nm)
can significantly alter the behavior of telecentric imaging system such
that the system no longer behaves in a telecentric manner. That is, the
measured image height of an object feature, such as the feature 25,
illuminated by peak wavelengths within the corrected range remains
substantially constant through the depth of field of the telecentric
imaging system 30. However, the measured image height of the same object
feature illuminated by peak wavelengths outside the corrected range
varies as a generally linear function of object distance through the
depth of field of the telecentric imaging system 30. From the perspective
of a filled aperture, the centroid of energy through any one image point
in the non-telecentric mode tends to extend at a fixed incline to the
substantially axially aligned centroid in the telecentric mode.

[0036] In a practical sense, telecentricity varies over a continuum
accommodating some tolerance for a minimal change in magnification over
the depth of field. Preferably, the telecentric imaging system 30 is at
least as telecentric as necessary for purposes of its intended operation
as a part of the measuring machine 10. In addition, the image results of
the telecentric imaging system 30 can be calibrated, such as by measuring
reticle projections at different object positions to compensate for
anticipated telecentric errors. The non-telecentric mode can be
referenced with respect to the telecentric mode as a either a reduction
in telecentricity or an increase in non-telecentricity whereby
magnification varies to a more significant degree over the depth of
field. That is, in the telecentric mode, the telecentric imaging system
30 meets the telecentricity requirements of the optical system, and in
the non-telecentric mode, the telecentric imaging system 30 deliberately
does not meet the telecentricity requirements of the optical system.
Larger departures in telecentricity between the telecentric and
non-telecentric modes can increase the accuracy by which relative object
displacements to the best focus position can be estimated but the
departures are preferably limited to avoid other unnecessary aberrations.

[0037] FIG. 2A provides a schematic depiction of the telecentric imaging
system 30 operating in a telecentric mode in which the chief ray 32 from
the highest point on a feature 34 (such as the feature 25) intended for
imaging extends substantially parallel to the optical axis 36. The
feature 34, which is shown in solid line at the nominal focus position
and in broken lines at opposite ends of the depth of field D, remains at
the same height h.sub.0 throughout the depth of field D. That is, the
height of the feature 34 as the feature 34 is thereby imaged does not
significantly vary through the depth of field D in which the feature 34
can be imaged with a desired level of sharpness.

[0038] FIG. 2B provides a schematic depiction of the telecentric imaging
system 30 operating in a non-telecentric mode in which the chief ray 42
from the highest point on the feature 34 intended for imaging extends at
an incline to the optical axis 36. As shown, the height of the feature 34
as the feature is thereby imaged varies from a height h.sub.a to a height
h.sub.b through the depth of field D in which the feature 34 can be
imaged with a desired level of sharpness.

[0039] The telecentric imaging system 30 arranged for operation in the
non-telecentric mode via a peak wavelength of illumination outside the
corrected range generally subjects imaging to a chromatic aberration
having an axial component in which the chief rays are focused at a
different distance along the optical axis and a lateral component in
which image points are focused in different positions within the image
field, thereby affecting magnification and/or a higher order form of
radial distortion. Particularly for this purpose, the telecentric imaging
system 30 includes at least one lens having refractive index that varies
as a function of wavelength, resulting, inter alia, in a change in focal
length. The aperture stop of the telecentric imaging system 30 operating
at a peak wavelength outside the corrected range is no longer located at
the back focus as required to locate an entrance pupil of the imaging
system at infinity. Thus, the image height or the size of a feature
within the image tends to vary with object distance through the depth of
field, as would be expected for an entocentric lens. Even at an ideal
focus position, a slight change in the magnification and/or higher order
distortion can be expressed by the lateral component of the chromatic
aberration.

[0040] While the images produced in the non-telecentric mode can include,
in addition to a change in magnification, higher order distortion,
particularly radial distortion, the variation in image height with object
distance at any point in the image field tends to remain linear. If the
lateral component of chromatic aberration is small enough to be ignored,
then an approximation of the best focus position can be predicted from a
difference in the measured image height of a feature and the expected
rate at which the change in image height varies with changes in object
distance to identify an amount of object displacement through the depth
of field required to reduce the change in image height to zero. To
incorporate the effects of lateral chromatic aberration, the target
change in image height can depart from zero to accommodate an expected
amount of magnification at the best focus position from the peak
wavelength outside the corrected range. Radial measures of image height
as a measure of size avoid some of the complexities associated with
higher order distortion affecting distance measurements between other
pairings of points in the image field and their relative rates of change
with object distance in the non-telecentric mode.

[0041] One approach to finding the best focus position includes measuring
a feature size, particularly as an image height, while operating the
measuring machine in a telecentric mode in which the illuminator is
operated at a peak wavelength within the corrected range. Image height
h.sub.0 can be measured from the center of the field to a given point on
an object feature, such as the feature 25. The measurement of image
height h.sub.0 must be taken within the depth of field of the telecentric
imaging system 30 at which the image height of the feature 25 can be
measured to desired accuracy and precision. While the object 24 is
located at the same position within the depth of field, the telecentric
imaging system 30 can be operated in a non-telecentric mode to take a
second measurement of image height h.sub.1 from the center of the field
to the given point on the feature 25. Preferably the change in mode from
telecentric to non-telecentric is effected by a change in the peak
wavelength of the illuminator from a peak wavelength within the corrected
range to a peak wavelength slightly outside the corrected range. Although
image formation is demonstrably non-telecentric, image sharpness is
preferably not significantly degraded over a substantially overlapping
depth of field.

[0042] A height difference .DELTA.h.sub.1 between the measured image
height h.sub.1 in the non-telecentric mode and the measured image height
h.sub.0 in the telecentric mode provides a local measure of the
distortion caused by the changed wavelength. As a percentage, the local
distortion can be represented as 100.times. (.DELTA.h.sub.1/h.sub.0). A
second measure of image height h.sub.2 in the non-telecentric mode can be
taken of the same feature 25 at a relatively displaced position of the
object 24 within the depth of field along the Z axis coincident with the
optical axis of the telecentric imaging system 30. Either the object 24,
the telecentric imaging system 30, or both can be moved relative to the
other to displace the object 24 with respect to the telecentric imaging
system 30 from a first position Z.sub.1 at which the first measurements
were taken to a second position Z.sub.2 by a predetermined, measured, or
otherwise known amount. Since the measured image height h.sub.0 taken in
the telecentric mode is not expected to change between the Z.sub.1 and
Z.sub.2 positions, a height difference .DELTA.h.sub.2 between the
measuring modes at the displaced Z.sub.2 position corresponds to the
difference between the measured image height h.sub.2 in the
non-telecentric mode at the Z.sub.2 position and the measured image
height h.sub.0 in the telecentric mode at the initial Z.sub.1 position. A
rate of change m.sub.r in image height with respect to a change in object
distance is given as follows:

where the subscript "r" references a radial position within the object
field corresponding to the image height h.sub.0.

[0043] Graphically depicted as a linear relation, FIG. 3 shows how a
further relative displacement of the object 24 through a distance
Z.sub.3-Z.sub.2 from the position Z.sub.2 to a position Z.sub.3 at which
the difference in measured image height between the telecentric and
non-telecentric modes is projected to equal zero can be found from the
following relationship:

Z 3 - Z 2 = - .DELTA. h 2 m r ( 2 )
##EQU00003##

[0044] An estimate of the distance Z.sub.3-Z.sub.1 from the position
Z.sub.1 to the position Z.sub.3 can be similarly found from the following
relationship:

Z 3 - Z 1 = - .DELTA. h 1 m r ( 3 )
##EQU00004##

[0045] The slope m.sub.r can be normalized to a slope M by dividing the
slope m.sub.r by the initial height h.sub.0 measured in the telecentric
mode. Any change in M throughout the image field in the non-telecentric
mode is attributable to higher order distortion. For calculating the
required displacement such as between Z.sub.1 and Z.sub.3 using the slope
M, the image height difference .DELTA.h.sub.1 can also be normalized by
dividing by h.sub.0 as a percent distortion.

[0046] By way of example, the telecentric imaging system 30 is considered
corrected for a peak wavelength of 530 nm, and thus operates in a
telecentric mode when the LEDs of the illuminator 14 are normally powered
to emit a peak wavelength of 530 nm. However, by adjusting the current to
the same LEDs of the illuminator 14, the peak wavelength emitted by the
LEDs can be increased to 540 nm, which is beyond the range through which
the telecentric imaging system is corrected. Therefore, the telecentric
imaging system 30 behaves in a non-telecentric manner as a result of
chromatic aberration. Although the departure in peak wavelength beyond
the corrected range affects the geometric positions of image points in
the image field as a form of magnification and higher order distortion,
the wavelength departure is limited to preserve image sharpness through
substantially the same depth of field. As such, the resulting distortions
are clearly measurable as changes in image feature size within the image
field.

[0047] The processor 28 is preferably arranged for controlling the shift
between telecentric and non-telecentric modes, such as by controlling the
current delivered to the LEDs of the illuminator 14. The processor 28 can
also be arranged to perform the various steps required to acquire the
desired size measurements in the two modes, measure or effect desired
relative displacements between the camera 12 and the object 24, access or
derive information concerning the function by which the size of the image
feature 25 changes through the depth of field, and perform the required
calculations for relating the measures of the size of the imaged feature
25 in the telecentric and non-telecentric modes to each other and to the
function by which the size of the imaged feature varies with the relative
displacement of the object 24 through the depth of field as an estimate
of a relative displacement of the object 24 from a best focus position.

[0048] At a first object position Z.sub.1 at an undetermined distance from
the best focus position along the Z axis within the depth of field, the
object feature 25 is measured by the measuring machine 10 operating in a
telecentric mode at 530 nm as having an image height h.sub.0 of 20
millimeters (mm), which is calibrated as the true height of the feature
25. Since the telecentric imaging system 30 operates in a telecentric
mode, the measured image height of the feature 25 is expected to remain
constant throughout the depth of field. With respect to the exemplary
measuring machine 10, this is same type of measurement for which the
measuring machine 10 is designed to take while operating for its intended
purpose. Thus, no special form of data collection or interpretation is
required to take this measure.

[0049] Although the distance from the best focus position is initially
undetermined, the Z.sub.1 position of the object 24 relative to the
telecentric imaging system 30 is preferably selected based on a known
relative position between the camera 12 and the table 26 and expected
dimensions of the object 24. The available information, which can be
drawn by the processor 28 from accessible memory, is preferably adequate
to initially relatively position the object 24 within the depth of field
and more preferably to relatively position the object 24 proximate to the
center of the depth of field.

[0050] At the same Z.sub.1 position, current to the LEDs of the
illuminator 14 is altered to shift the peak wavelength emitted by the
LEDs to 540 nm for operating the same telecentric imaging system 30 in a
non-telecentric mode. The image height measurement h.sub.1 of the same
feature 25 at the same Z.sub.1 position is returned at 19.99677499 mm,
yielding a difference .DELTA.h.sub.1 of -0.0032250120 mm. While the peak
wavelength can be displaced in other ways such as by filtering, the shift
in wavelength, as a way to operate the telecentric imaging system 30 in a
non-telecentric mode, is preferably carried out without mechanically
displacing parts of the imaging system so that the telecentric imaging
system 30 is otherwise unaffected by the change and can be restored to
its normal telecentric operating mode without requiring realignment or
other corrective actions associated with mechanical displacements. The
illuminator 14, such as the depicted ring light, also preferably remains
fixed with respect to the telecentric imaging system 30 to further reduce
variability between measurement modes so that the departure from
telecentricity is limited to the intended shift in wavelength.

[0051] The object 24 can be relatively shifted by a known amount to a
second position Z.sub.2 within the depth of field, such as by translating
the carriage 16 together with the camera 12 along the vertical Z-axis and
measuring the displacement. The relative displacement is preferably a
predetermined amount that is expected to be within the depth of field.
The measurement of the displacement along the vertical Z-axis, which can
provide feedback for confirming the desired displacement, is also
preferably a conventional feature of the measuring machine 10. Given a
relatively large depth of field in the vicinity of .+-.50 mm, a
displacement of 10 mm from the Z.sub.1 position is expected to remain
within the depth of field. However, if the shift relatively positions the
object feature 25 beyond the depth of field, i.e., beyond the range of
measurement qualified for the machine 10, an opposite direction of
displacement along the Z-axis can be effected to secure a second
measurement in the non-telecentric mode.

[0052] At a vertical displacement of the carriage 16 by 10 mm, a second
image height measurement h.sub.2 of the feature 25 is taken in the
non-telecentric mode with the peak wavelength emitted by the LEDs
remaining at 540 nm. A value of h.sub.2 is returned at 19.99570021
yielding a height difference .DELTA.h.sub.2 with respect to the
telecentric mode of -0.0042997860 mm. The greater height difference
suggests that the displacement from Z.sub.1 to Z.sub.2 was in the wrong
direction with respect to the best focus position.

[0053] The change in height difference .DELTA.h.sub.2-.DELTA.h.sub.1 of
-0.001074774 mm, which is equivalent to the change in height
h.sub.2-h.sub.1, with respect to the relative change in object distance
Z.sub.2-Z.sub.1 of 10 mm yields a slope m.sub.r of -0.0001074774.
Assuming that the local change in magnification varies linearly through
the depth of focus in the non-telecentric operating mode, an estimated
displacement from the closer initial Z.sub.1 position is found by
dividing the negative of the height difference .DELTA.h.sub.1 of
0.0032250120 mm by the slope m.sub.r of -0.0001074774 yielding a
displacement Z.sub.3-Z.sub.1 of -30.00642 mm to the position at which the
height difference .DELTA.h.sub.a between the telecentric and
non-telecentric measurements is equal to zero.

[0054] The estimated displacement of -30.00642 mm is actually 0.00642 mm
or approximately 6 microns (.mu.m) from a -30.000 mm displacement to the
actual best focus position for the given example. This accuracy is
sufficient to maintain the object 24 within the depth of field through
virtually any practical change in the magnification of the telecentric
imaging system. Although a lateral component of the chromatic aberration
produced at the shifted wavelength is expected to affect image size at
the best focus position negating the assumption that the measured image
heights should be the same between the telecentric and non-telecentric
modes at the best focus position, the small 10 nm difference in
wavelength is not expected result in a significant image height
difference of the measured feature between the two measurement modes at
the best focus position. In fact, the 10 nm difference in peak wavelength
is still expected to be within the bandwidth of a typical LED emitter
useful for machine vision systems.

[0055] Instead of relatively displacing the object 24 along the Z axis for
taking a second measurement in the non-telecentric mode, the approximate
slope m.sub.r for a given image height can be determined by optically
modeling the telecentric imaging lens 30 using modeling software such as
a ray tracing algorithm such as, but not limited to, Zemax.RTM. software
from Zemax, LLC of Redmond, Wash. or CODE V.RTM. optical design software
from Synopsys, Inc. of Pasadena, Calif., or by earlier recorded
measurements between relatively displaced positions of another object.
Different slopes m.sub.r can be stored for different radial zones "r" of
the image field to accommodate the effects of higher order radial
distortion. However, where higher order distortion is known to be minimal
or only a more general approximation of the best focus position is
required, a single normalized slope M can be used to define the rate of
change in magnification with object distance through the depth of field
in the non-telecentric mode.

[0056] In the Z.sub.1 position, a single measurement can be taken in the
telecentric mode to measure the image height h.sub.0 of the object
feature 25 and a single measurement can be taken in the non-telecentric
mode to measure the image height h.sub.1 of the same object feature 25. A
plurality of slopes m.sub.r corresponding to different radial zones of
the image field can be stored in a lookup table accessible from memory by
the processor 28 or otherwise mathematically referenced (e.g., a fitted
equation) to provide a value for the slope m.sub.r at the image height
h.sub.0. Given the difference in image height .DELTA.h.sub.1
corresponding to the height difference h.sub.1-h.sub.0 and the slope
m.sub.r corresponding to a rate of change of image height with relative
displacement of the object 24 along the Z axis within the depth of field,
an amount of relative object displacement Z.sub.2-Z.sub.1 can be
predicted for approaching the best focus position as follows:

Z 2 - Z 1 = - .DELTA. h 1 m r ( 4 )
##EQU00005##

[0057] If desired, a second measurement can be made in the non-telecentric
mode at the predicted best focus position to confirm that the measured
height difference between the two modes is minimal or otherwise
corresponds to a height difference expected as a result of lateral
chromatic aberration. Particularly for wavelengths that depart more
significantly from the corrected range, the expected height differences
between the two modes at the best focus position as a lateral component
of chromatic aberration can also be stored and accessed for better
estimating the amount of relative object displacement required to reach
the best focus position.

[0058] If the measured image height difference .DELTA.h.sub.2 between the
two modes at the first estimated position Z.sub.2 of the best focus
differs significantly from zero or from the height difference expected as
a result of lateral chromatic aberration, a second empirically derived
estimate can be calculated for estimating the remaining relative object
displacement Z.sub.3-Z.sub.2 to the best focus position. Similar to the
example above in which two image height measurements are taken in the
non-telecentric mode at different relative object positions Z.sub.1 and
Z.sub.2, a new empirically defined slope m.sub.r2 can be calculated as
follows:

[0059] Based on this new slope a further estimate of the remaining
displacement Z.sub.3-Z.sub.2 to the best focus position defined as a
position at which the measures of image height match between the two
operating modes can be calculated according to the following
relationship:

Z 3 - Z 2 = - .DELTA. h 2 m r 2 (
6 ) ##EQU00007##

[0060] However, if a significant difference in the measures of image
height between the two modes at the best focus position is expected as a
result of lateral chromatic aberration .delta.h.sub.r, such as derived
from modeling or prior measurements and scaled to radial position in the
image field, the calculated displacement Z.sub.3-Z.sub.2 can be found as
follows:

Z 3 - Z 2 = .delta. h r - .DELTA. h 2
m r 2 ( 7 ) ##EQU00008##

[0061] Assuming that a change in magnification is the dominate geometric
change between relative object displacements in the non-telecentric mode,
the normalized slope M characterizing the entire image field can be used
in a first estimate of the best focus position following the two modes of
measurement at an initial relative object position Z.sub.2. The effect of
any significant lateral chromatic aberration can also be similarly
normalized throughout the field as .delta.H corresponding to a simple
difference in magnification between the modes at the best focus position.
Thus, a first estimate of relative object displacement Z.sub.2-Z.sub.1
can be calculated as follows:

Z 2 - Z 1 = .delta. H - .DELTA. h 1 h 0
M ( 8 ) ##EQU00009##

[0062] Although the use of the normalized values for the slope M and/or
the offset .delta.H provides a coarser estimate of the best focus
position than the estimates based on the slopes m.sub.r and offsets
.delta.h.sub.r that are scaled to different radial positions in the image
field, the coarser estimates may be sufficiently accurate in some
situations to relatively position the object 24 near enough to the best
focus position for operating the measuring machine 10 as desired. For
example, the coarser estimate could be sufficiently accurate to
accommodate changes in the magnification of the telecentric imaging
system (e.g. 10 times the lower magnification) that compress the depth of
field by a factor of 10 or more without requiring any finer adjustment of
the focus position to remain within the narrower depth of field.

[0063] A second measure of image height can be taken in the
non-telecentric mode at the estimated focus position Z.sub.2 to confirm
the accuracy of the estimate. If the accuracy is not sufficient, an
empirically defined slope m.sub.r can be calculated from the measurements
taken in the two relatively displaced object positions Z.sub.1 and
Z.sub.2, and the calculated slope m.sub.r can be used to calculate a
further relative object displacement Z.sub.3-Z.sub.2 to the best focus
position as described above.

[0064] Although a single measurement of image height or size in the
telecentric mode at any one relative object position can be
representative of image height or size measurements of the same feature
at different relative object positions in the telecentric mode,
additional measurements can be made of the same object feature at
different relative object positions in the telecentric mode for purposes
of confirmation or potentially improved accuracy.

[0065] Instead of shifting the peak wavelength of the same light source
between (a) a peak wavelength within the corrected range and (b) a peak
wavelength outside the corrected range, different wavelength light
sources (e.g., different color LEDs or LEDs with different bin numbers)
could be used for purposes of illumination in the telecentric and
non-telecentric modes. That is, one light or set of lights could be used
in the telecentric mode, and another light or different set of lights
could be used in the non-telecentric mode. The wavelength difference
between the modes can be increased, particularly for estimating the best
focus positions of telecentric imaging systems with narrow depths of
field, so long as the measurements of image height in the non-telecentric
mode can be taken with the required accuracy. While the wavelengths
generated by the light sources are generally expected to be within the
visible range, wavelengths beyond the visible range can also be used,
where the refractive index of one or more lens elements in the
telecentric imaging system varies with a change in the wavelength to
support the operation of the telecentric imaging system in a
non-telecentric mode.

[0066] In addition to varying the wavelength of illumination, other
changes can be made to operate an otherwise telecentric imaging system in
a non-telecentric mode. For example, the refractive index of one or more
optical components, portions of optical components, or a medium between
the optical components can be altered so that the telecentric imaging
system is no longer telecentric and, as such, measured image height
varies with object distance through the depth of field. Some optical
materials exhibit a change in refractive index when exposed to an
electric field. For example, certain crystalline solids exhibit a
so-called Pockels effect and other optical materials can be effectively
subject to a so-called Kerr effect. Temperature and pressure are also
known to affect the refractive index of optical materials. Although the
change between telecentric and non-telecentric modes is preferably
accomplished without mechanical motion to avoid alignment or hysteresis
issues, electrically focus-tunable lenses are available that alter lens
radii in response to a control current. An optic could also be moved into
and out of the optical path or between positions along the optical path
of the telecentric imaging system to switch between telecentric and
non-telecentric modes.

[0067] While the methods and apparatus described above generally provide
for estimating a best focus position between a telecentric lens and an
object intended for imaging by the lens, the estimate can be useful for
other purposes beyond relatively adjusting the object to the best focus
position. For example, the estimate can also be useful as a range finder
to determine the distance of the object from the lens, such as for
assuring adequate clearance exists for relatively moving or removing the
object.

[0068] While the contemplated optical systems include a telecentric
imaging system convertible for use in both a telecentric and
non-telecentric mode and a sensor for capturing image information, the
systems also preferably provide for measuring the imaged height or size
of an object feature, for relatively displacing the object with respect
to the telecentric imaging system along a Z axis extending through a
depth of field, and for measuring or otherwise gaging the amount of
relative motion along the Z axis. Preferably, the contemplated optical
systems are a part of an optical measuring machine already equipped with
a telecentric metrology camera, a motorized Z axis of measurement, and a
processor operating under the control of algorithms for interpreting the
image height or size of object features imaged by the camera. Other than
operating their telecentric cameras in a non-telecentric mode, the
optical measuring machines can be operated for taking measurements as
originally designed and calibrated while carrying out procedures
described above for estimating the best focus position for the machine.
In accordance with the usual programming of such machines, the procedures
can be carried out automatically or semi-automatically by invoking
pre-programmed routines. One example of a measuring machine series
especially suited to the purposes herein is a SNAP large-field-of-view
digital measuring machine from Optical Gaging Products, manufactured in
Rochester, N.Y.

[0069] Although the object intended for reference to the best focus
position preferably contains imageable features subject to measurement
through the telecentric imaging system, highly reflective parts with
little contrast can be referenced relative to the best focus position by
projecting a grid onto the object and by measuring imaged features (e.g.,
points or lines) of the grid.

[0070] Variations of the above disclosed embodiments, features within the
embodiments, and suggested alternatives will be apparent to those of
skill in the art as well as their application in others systems and
environments in accordance with the teaching provided for this invention.